Steps to replicate:
1. Create an assignment activity with the settings:

• Students submit in groups - Yes
• Require all group members submit - No

2. As a student, submit a pdf assignment file.
3. Grade this group submission, and include an annotated PDF while grading
4. When returning to the "View/grade all submissions" page you get the following:

Error writing to database

More information about this error
Debug info: Column 'attemptnumber' cannot be null
INSERT INTO mdl_assign_grades (assignment,userid,timecreated,timemodified,grade,grader,attemptnumber) VALUES(?,?,?,?,?,?,?)
[array (
0 => '1',
1 => '3',
2 => 1409875018,
3 => 1409875018,
4 => -1,
5 => '2',
6 => NULL,
)]
Error code: dmlwriteexception
Stack trace:

line 446 of /lib/dml/moodle_database.php: dml_write_exception thrown
line 1080 of /lib/dml/mysqli_native_moodle_database.php: call to moodle_database->query_end()
line 1122 of /lib/dml/mysqli_native_moodle_database.php: call to mysqli_native_moodle_database->insert_record_raw()
line 2667 of /mod/assign/locallib.php: call to mysqli_native_moodle_database->insert_record()
line 679 of /mod/assign/feedback/editpdf/classes/document_services.php: call to assign->get_user_grade()
line 69 of /mod/assign/feedback/editpdf/locallib.php: call to assignfeedback_editpdf\document_services::get_feedback_document()
line 240 of /mod/assign/feedback/editpdf/locallib.php: call to assign_feedback_editpdf->get_widget()
line 220 of /mod/assign/feedback/editpdf/locallib.php: call to assign_feedback_editpdf->view()
line 1100 of /mod/assign/gradingtable.php: call to assign_feedback_editpdf->view_summary()
line 1209 of /mod/assign/gradingtable.php: call to assign_grading_table->format_plugin_summary_with_link()
line 729 of /lib/tablelib.php: call to assign_grading_table->other_cols()
line 429 of /mod/assign/gradingtable.php: call to flexible_table->format_row()
line 1330 of /lib/tablelib.php: call to assign_grading_table->format_row()
line 1442 of /lib/tablelib.php: call to table_sql->build_table()
line 1185 of /mod/assign/renderer.php: call to table_sql->out()
line 981 of /mod/assign/renderer.php: call to mod_assign_renderer->flexible_table()
line 221 of /lib/outputrenderers.php: call to mod_assign_renderer->render_assign_grading_table()
line 3082 of /mod/assign/locallib.php: call to plugin_renderer_base->render()
line 3114 of /mod/assign/locallib.php: call to assign->view_grading_table()
line 505 of /mod/assign/locallib.php: call to assign->view_grading_page()
line 53 of /mod/assign/view.php: call to assign->view()

This seems to be an issue caused by there being a record for the user with this assignment id in the mdl_assign_grades table, but there is no corresponding record in the mdl_assign_submission table.